MadSci Network: Engineering
Query:

Re: Why cann't the airlines control cabin pressure better so no ear pain?

Date: Tue Feb 22 20:08:14 2000
Posted By: Madhu Siddalingaiah, Physicist, author, consultant
Area of science: Engineering
ID: 949498394.Eg
Message:

Hi Paul.

That's a good question. Aircraft are capable of maintaining accurate cabin pressure, but this is problematic for structural reasons. Two of the very first commercial jet liners mysteriously crashed, killing all on board. The cause of these crashes was determined to be stresses on the fuselage from repeated pressure variations. The DeHavilland Comet was a revolutionary aircraft, but suffered from subtle design flaws. The flaws were corrected in later aircraft like the the Boeing 707. You can read about the Comet here.

The stress on the fuselage of an aircraft is a function of the difference in pressure between the inside of the cabin and the outside. If the cabin is not pressurized, there would be no stress due to pressure differences. This is fine for low flying aircraft, but unnacceptable for flight at altitudes of 10000 feet above sea level. Above this altitude, hypoxia and possibly even death in some elderly passengers could result. One solution is to supply oxygen to all the passengers all the time, but this is impractical.

If the cabin was pressurized to exactly that of sea level (or the pressure of the originating airport), excessive stress on the fuselage would result. At 30000 feet, atmospheric pressure is almost a quarter of the pressure at sea level. Sea level cabin pressure would introduce about 10 pounds per square inch on the fuselage. Holding that much pressure would require a very thick body, not to mention the windows! The end result would be a very heavy aircraft that would yield low range and fuel economy. A really cool atmospheric pressure calculator can be found here .

Aerospace engineers make a compromise. They pressurize the cabin of airliners, but they choose a pressure much less than that of sea level. This reduces the stress on the fuselage and eliminates the need for continuous oxygen. Of course, this introduces some discomfort to the passengers, but that's the tradeoff. There is a product on the market called "Earplanes" which helps in reducing ear pain at high altitudes.

There is also the problem of different field elevations of airports. The field elevation of the departure airport will probably not be the same as that of the arrival airport. Let's say you left from Washington, DC and travel to Denver, Colorado. The field elevation of all of the major airports in the Washington metro area are less than 1000 feet above sea level. Airports in the Denver area are more than 5000 feet above sea level, the pressure is quite different. Ideally, the cabin pressure within the aircraft should slowly shift towards the destination, but the gain is not worth the cost to the airlines, so they don't do it.

I hope this answers your question.


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